How to show more digits in blender when working at a small scale
In the Units section, select not only metric, but change the scale to .001 to work in milimiters instead of meters.



You should be able to work numerically in a very small scale then.



Doing operations using keyboard shortcuts works now at the scale you've set, meaning that if you want to move something 2mm in the X axis, for example, you'd press GX2

To make navigating and manipulation easier, remember to set the scale of the 3D viewport as well.